tailscale/disco/disco.go

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// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package disco contains the discovery message types.
//
// A discovery message is:
//
// Header:
// magic [6]byte // “TS💬” (0x54 53 f0 9f 92 ac)
// senderDiscoPub [32]byte // nacl public key
// nonce [24]byte
//
// The recipient then decrypts the bytes following (the nacl secretbox)
// and then the inner payload structure is:
//
// messageType byte (the MessageType constants below)
// messageVersion byte (0 for now; but always ignore bytes at the end)
// message-paylod [...]byte
package disco
import (
"encoding/binary"
"errors"
"fmt"
"net"
"inet.af/netaddr"
)
// Magic is the 6 byte header of all discovery messages.
const Magic = "TS💬" // 6 bytes: 0x54 53 f0 9f 92 ac
// NonceLen is the length of the nonces used by nacl secretboxes.
const NonceLen = 24
type MessageType byte
const (
TypePing = MessageType(0x01)
TypePong = MessageType(0x02)
TypeCallMeMaybe = MessageType(0x03)
)
const v0 = byte(0)
var errShort = errors.New("short message")
// Parse parses the encrypted part of the message from inside the
// nacl secretbox.
func Parse(p []byte) (Message, error) {
if len(p) < 2 {
return nil, errShort
}
t, ver, p := MessageType(p[0]), p[1], p[2:]
switch t {
case TypePing:
return parsePing(ver, p)
case TypePong:
return parsePong(ver, p)
case TypeCallMeMaybe:
return CallMeMaybe{}, nil
default:
return nil, fmt.Errorf("unknown message type 0x%02x", byte(t))
}
}
// Message a discovery message.
type Message interface {
// AppendMarshal appends the message's marshaled representation.
AppendMarshal([]byte) []byte
}
// appendMsgHeader appends two bytes (for t and ver) and then also
// dataLen bytes to b, returning the appended slice in all. The
// returned data slice is a subslice of all with just dataLen bytes of
// where the caller will fill in the data.
func appendMsgHeader(b []byte, t MessageType, ver uint8, dataLen int) (all, data []byte) {
// TODO: optimize this?
all = append(b, make([]byte, dataLen+2)...)
all[len(b)] = byte(t)
all[len(b)+1] = ver
data = all[len(b)+2:]
return
}
type Ping struct {
TxID [12]byte
}
func (m *Ping) AppendMarshal(b []byte) []byte {
ret, d := appendMsgHeader(b, TypePing, v0, 12)
copy(d, m.TxID[:])
return ret
}
func parsePing(ver uint8, p []byte) (m *Ping, err error) {
if len(p) < 12 {
return nil, errShort
}
m = new(Ping)
copy(m.TxID[:], p)
return m, nil
}
// CallMeMaybe is a message sent only over DERP to request that the recipient try
// to open up a magicsock path back to the sender.
//
// The sender should've already sent UDP packets to the peer to open
// up the stateful firewall mappings inbound.
//
// The recipient may choose to not open a path back, if it's already
// happy with its path. But usually it will.
type CallMeMaybe struct{}
func (CallMeMaybe) AppendMarshal(b []byte) []byte {
ret, _ := appendMsgHeader(b, TypeCallMeMaybe, v0, 0)
return ret
}
// Pong is a response a Ping.
//
// It includes the sender's source IP + port, so it's effectively a
// STUN response.
type Pong struct {
TxID [12]byte
Src netaddr.IPPort // 18 bytes (16+2) on the wire; v4-mapped ipv6 for IPv4
}
const pongLen = 12 + 16 + 2
func (m *Pong) AppendMarshal(b []byte) []byte {
ret, d := appendMsgHeader(b, TypePong, v0, pongLen)
d = d[copy(d, m.TxID[:]):]
ip16 := m.Src.IP.As16()
d = d[copy(d, ip16[:]):]
binary.BigEndian.PutUint16(d, m.Src.Port)
return ret
}
func parsePong(ver uint8, p []byte) (m *Pong, err error) {
if len(p) < pongLen {
return nil, errShort
}
m = new(Pong)
copy(m.TxID[:], p)
p = p[12:]
m.Src.IP, _ = netaddr.FromStdIP(net.IP(p[:16]))
p = p[16:]
m.Src.Port = binary.BigEndian.Uint16(p)
return m, nil
}
// MessageSummary returns a short summary of m for logging purposes.
func MessageSummary(m Message) string {
switch m := m.(type) {
case *Ping:
return fmt.Sprintf("ping tx=%x", m.TxID)
case *Pong:
return fmt.Sprintf("pong tx=%x", m.TxID)
case CallMeMaybe:
return "call-me-maybe"
default:
return fmt.Sprintf("%#v", m)
}
}